Genetics unit 1
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The fundamental physical unit of heredity, whose existence can be confirmed by allelic variants and which occupies a specific chromosomal locus. A DNA sequence coding for a single polypeptide.
One of the possible mutational forms of a gene, which is often distinguished from other alleles by phenotypic effects.
The complex of DNA, RNA, histones, and nonhistone proteins that make up uncoiled chromosomes, characteristic of the eukaryotic interphase nucleus.
In prokaryotes, a DNA molecule containing the organism’s genome; in eukaryotes, a DNA molecule complexed with RNA and proteins to form a threadlike structure containing genetic information arranged in a linear sequence and visible during mitosis and meiosis.
Define Homologous Chromosome
Chromosomes that synapse or pair during meiosis and which are identical with respect to their genetic loci and centromere placement.
A cell or organism having one member of each pair of homologous chromosomes. Also, the gametic chromosome number.
A condition in which each chromosome exists in pairs; having two of each chromosome.
The heterochromatic terminal region of a chromosome.
The specialized heterochromatic chromosomal region at which sister chromatids remain attached after replication, and the site to which spindle fibers attach to the chromosome during cell division. Location of the centromere determines the shape of the chromosome during the anaphase portion of cell division. Also known as the primary constriction.
A fibrous structure with a size of about 400 nm, located within the centromere. It appears to be the site of microtubule attachment during division.
Region of the cytoplasm containing a pair of centrioles.
The process of cell division in gametogenesis or sporogenesis during which the diploid number of chromosomes is reduced to the haploid number.
The separation of chromosomes during the anaphase stage of cell division.
A cell division error in which homologous chromosomes (in meiosis) or the sister chromatids (in mitosis) fail to separate and migrate to opposite poles; responsible for defects such as monosomy and trisomy.
The sequence of growth phases of an individual cell, divided into G1 (gap 1), S (DNA synthesis), G2 (gap 2), and M (mitosis). A cell may temporarily or permanently be withdrawn from the cell cycle, in which case it is said to enter the G0 stage.
Synapsed homologous chromosomes in the first prophase of meiosis.
The products of tetrad separation or disjunction at meiotic prophase I. Each dyad consists of two sister chromatids joined at the centromere.
The four chromatids that make up paired homologs in the prophase of the first meiotic division. In eukaryotes with a predominant haploid stage (some algae and fungi), tetrad denotes the four haploid cells produced by a single meiotic division.
Cytoplasmic fibrils formed during cell division that are involved with the separation of chromatids at the anaphase stage of mitosis and meiosis as well as their movement toward opposite poles in the cell.
The division or separation of the cytoplasm during mitosis or meiosis.
Tumor Suppressor Genes
A gene that encodes a product that normally functions to suppress cell division. Mutations in tumor-suppressor genes result in the activation of cell division and tumor formation.
A gene that functions to initiate, facilitate, or maintain cell growth and division. Proto-oncogenes can be converted to oncogenes by mutation.
In eukaryotic cells, a class of proteins that are synthesized and degraded in synchrony with the cell cycle and regulate passage through stages of the cycle.
The chromosome division that halves the number of centromeres and thus reduces the chromosome number by half. The first division of meiosis is a reductional division. See equational division.
A division stage where the number of centromeres is not reduced by half, but where each chromosome is split into longitudinal halves that are distributed into two daughter nuclei. Chromosome division in mitosis and the second meiotic division are examples of equational divisions. See reductional division.
A pair of crosses in which the genotype of the female in one is present as the genotype of the male in the other, and vice versa.
A cross between an individual whose genotype at one or more loci may be unknown and an individual who is homozygous recessive for the genes in question.
A genetic cross involving only one character (e.g., AA x aa).
A genetic cross involving two characters in which the parents possess different forms of each character (e.g., yellow, round x green, wrinkled peas).
An individual who is the focus of, or who called attention to, the genetic study leading to the production of a pedigree tracking the inheritance of a genetically determined trait of interest. Formerly known as a propositus.
Null Hypothesis (H0)
Used in statistical tests, the hypothesis that there is no real difference between the observed and expected data sets. Statistical methods such as chi-square analysis are used to test the probability associated with this hypothesis.
The most commonly observed phenotype or genotype, designated as the norm or standard.
Loss of Function
Mutations that produce alleles with reduced or no function.
Gain of Function
A mutation that produces a phenotype different from that of the normal allele and from any loss of function alleles.
A mutant allele that produces no functional gene product. Usually inherited as a recessive trait.
The changes that occur in the frequency of alleles and genotypes in populations as a result of differential reproduction.
Expressing a heterozygous phenotype that is distinct from the phenotype of either homozygous parent. Also called partial dominance.
Condition in which the phenotypic effects of a gene’s alleles are fully and simultaneously expressed in the heterozygote.
In a population of organisms, three or more alleles of the same gene.
The transmission of a phenotypic trait whose expression depends on the additive effect of a number of genes.
Nonreciprocal interaction between nonallelic genes such that one gene influences or interferes with the expression of another gene, leading to a specific phenotype.
A genetic test to determine whether two mutations occur within the same gene (or cistron). If two mutations are introduced into a cell simultaneously and produce a wild-type phenotype (i.e., they complement each other), they are often nonallelic. If a mutant phenotype is produced, the mutations are noncomplementing and are often allelic.
A trait that is expressed in only one sex even though the trait may not be X-linked.
Sex Influenced Traits
Phenotypic expression that is conditioned by the sex of the individual. A heterozygote may express one phenotype in one sex and the alternate phenotype in the other sex, e.g., pattern baldness in humans.
The frequency, expressed as a percentage, with which individuals of a given genotype manifest at least some degree of a specific mutant phenotype associated with a trait.
The degree or range in which a phenotype for a given trait is expressed.
The study of modifications in an organism’s gene function or phenotypic expression that are not attributable to alterations in the nucleotide sequence (gene mutation) of the organism’s DNA.
Enzymatic transfer of methyl groups from S-adenosylmethionine to biological molecules, including phospholipids, proteins, RNA, and DNA. Methylation of DNA is associated within the regulation of gene expression and with epigenetic phenomena such as imprinting.
The process by which the expression of a gene depends on whether it has been inherited from a male or a female parent. Also referred to as parental imprinting.
The phenomenon in which the severity of the symptoms in genetic disorders increases from generation to generation and the age of onset decreases from generation to generation. It is caused by the expansion of trinucleotide repeats within or near a gene and was first observed in myotonic dystrophy.
Having a gene present in a single dose in an otherwise diploid cell. Usually applied to genes on the X chromosome in heterogametic males.
A mutation that is expressed only under a certain condition, i.e., a wild-type phenotype is expressed under certain (permissive) conditions and a mutant phenotype under other (restrictive) conditions.
A mutant microorganism or cell line that requires a nutritional substance for growth that can be synthesized, and is not required, by the wild-type strain.
A strain (usually of a microorganism) that is capable of growth on a defined, minimal medium. Wild-type strains are usually regarded as prototrophs, and contrasted with auxotrophs.
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